1. Field of the Invention
This invention relates to a time-temperature indicator and, more particularly, to an indicator that is inactive until it is activated by actinic radiation.
2. Description of the Prior Art
Several patents have disclosed the use of color-changing indicators to monitor the time-temperature history of perishables. Among these are U.S. Pat. No. 4,189,399, issued Feb. 19, 1980, to Patel; and U.S. Pat. No. 4,212,153, issued July 15, 1980, to Kydonieus et al.
When the perishable to be monitored has a short useful lifetime and/or requires refrigeration, then it is desirable, if not essential, to use an indicator that is inactive untial activated. Patel, U.S. Pat. Nos. 4,208,186, issued June 17, 1980, and 4,276,190, issued June 30, 1981, disclosed diacetylenic compositions having an inactive form that is activated by contact with an activating vapor. Activation of a diacetylenic monomer in salt form by conversion to the acid form was disclosed in U.S. Pat. No. 4,373,032, issued Feb. 9, 1983, to Preziosi et al.
U.S. Pat. No. 3,768,976, issued Oct. 30, 1973, to Hu et al., has disclosed a temperature-time integrating indicator that is based on temperature-dependent oxygen diffusion into a package that includes an aqueous solution of a redox dye. The dye is dark in the reduced state and becomes colorless when it is oxidized. A similar indicator, involving a free radical sensitive dye and a peroxide on a carrier, was disclosed in U.S. Pat. No. 3,966,414, issued June 29, 1976, to Khattab et al.
Photoactivation of a variety of chemical processes has been reported. It is known, for example, that certain onium salts are photoinitiators of cationic polymerization (see, e.g., J. V. Crivello, Polymer Eng. and Sci. 23, 953 (1983); and J. V. Crivello et al., J. Polymer Sci., Symposium No. 56, 383 (1976)).
Photogeneration of a hydrohalic acid has been disclosed by S. Maslowski, Appl. Optics 13, 857 (1974) and in U.S. Pat. No. 4,247,611, issued Jan. 27, 1981, to Sander et al.